docs: def coalesce(self, npartitions):
          """
          Coalesce data (Spark only).
  
          Parameters
          ----------
          npartitions : int
              Number of partitions after coalescing.
          """
          if self.mode == 'spark':
              current = self.values.tordd().getNumPartitions()
              if npartitions > current:
                  raise Exception('Trying to increase number of partitions (from %g to %g), '
                                  'cannot use coalesce, try repartition' % (current, npartitions))
              self.values._rdd = self.values._rdd.coalesce(npartitions)
              return self
          else:
              notsupported(self.mode)